I have a step daughter who had had sex at 14, however the first time she did it, she used protection and did it in her bedroom. She said she wouldn't do it again as she regretted it. Now 6 months on, she has done it again with another boy unprotected, however when I asked her why she did it again, she said becuase she wanted too.
The most disappointing thing was she was unprotected, especially after the last time caused so much grief. But at the end of the day, she obviously wants to do it, and there is no amount of telling her that will change her mind.
So I think as long as both parties understand they are doing a very adult thing and should protect themselves and each other, then I don't think you can stop them.
I did it at 14, with a boy that I was seeing for a while and it is young, but I think the emphasis should be on talking to them so if they have any concerns and also educating them.
She is now going on the pill, and has promised that if it isn't covered up it doesn't happen!!
However, I did feel like hitting the roof, but realised that I needed to keep my cool because I know if she can talk to me, she will get better advice then getting it from her friends.